An electric furnace of arc type normally comprises a hearth having an enclosure formed by a side wall and a removable roof above the hearth, the roof being removed so the hearth can be charged with solid metal, such as scrap. After charging, the roof is replaced and one or more arcing electrodes are lowered through suitable openings formed in the roof, these electrodes serving first to melt down the solid metal and form a melt in the hearth and, thereafter, to heat the melt. The furnace can be tilted in opposite directions from a normal vertical position, and on opposite sides of the furnace's side wall, a deslagging port and a tapping port and spout, respectively, are positioned. These are permanent openings required for the normal operation of the furnace, the furnace being tilted towards the deslagging port for the removal of slag floating on the melt formed in the hearth, and, for tapping, being tilted towards the tapping port. The hearth enclosure's side wall may have other permanent openings, but the ports specifically mentioned are mandatory for all practical purposes. In the case of an arc furnace powered by alternating current, two or more arcing electrodes are used, depending on whether the power source is single or multi-phase, so there is no problem in connection with powering the electrodes for the melt-down of the initially charged solid metal or scrap.
However, in the case of a DC arc furnace it is usually necessary to provide some kind of hearth connection with one polarity of current, while the arcing electrode or electrodes are powered with the opposite polarity. Preferably the arcing electrodes are operated as cathodes, the hearth connection, therefore, being operated as the anode.
Various hearth connections are known and the hearth itself can be made of graphite, but this is extremely expensive. Therefore, it is preferable to make the hearth in the normal manner of electrically non-conductive refractory, although this requires the hearth connection to make electrical contact with a melt on the hearth throughout only a localized portion of the melt. Hearth connections of this type are known, a particularly advantageous form being disclosed by the Stenkvist U.S. Patent application Ser. No. 587,714, filed June 17, 1975. This localized contact-type of hearth connection, contacting the melt at what is essentially only a point compared to the overall hearth area, has the disadvantage that it is insufficient, if not ineffective, in getting adequate arcing power into an initially loaded charge of solid metal or scrap, to permit the arcing electrode or electrodes to melt down the charge to a melt on the hearth. The described type of hearth connection is fully effective only in the case of a melt in the hearth.
To overcome the problem of effecting a melt-down of solid metal or scrap in a DC arc furnace, the prior art has proposed the use of one or more starting electrodes which can temporarily be pushed into contact with the charge firmly to provide an effective contact with the charge during the melt-down, the starting electrode or electrodes then being withdrawn so that the thus-formed melt, effected by the arcing electrode or electrodes, can be processed and ultimately tapped. However, insofar as is known, the prior art has been silent concerning the manner of using the starting electrode or electrodes in a commercially practical manner.
The construction and operation of any electric arc furnace is inherently expensive. If the furnace has to be designed to accommodate one or more starting electrodes, usually proposed for insertion through the roof of the furnace in the manner of the normal arcing electrodes, that expense is increased to a commercially impractical degree. In addition to the normal arcing electrode openings through the roof, which are already expensive because they must be provided with water-cooled rings, through which the electrodes extend, and already permit the escape of the start-up explosion noise and smoke, at least one additional opening through the furnace roof must be provided. Being directly above the hearth where the melt-down is being effected, such troubles are substantially increased.
With the foregoing in mind, the object of the present invention is to provide a DC arc furnace with an arrangement for the use of at least one starting electrode, free from the objections connected with inserting the starting electrode through the furnace roof, and which does not require any modification of the usual electric furnace construction while permitting enjoyment of the advantages of what might be called the point-contact type of hearth connection and, particularly, the very effective form of such a type, disclosed by the previously-referred-to patent application.